Aquatic Environments

The lacustrine (lake) environment can be divided into categories by total depth and by the depth within the water column, by light availability, and by habitats. The shallow environment along the shore where rooted aquatic plants (including completely submerged plants) can grow is called the littoral zone. Deeper water is called the limnetic or pelagic zone. The upper layer reached by sunlight and therefore dominated by phytoplankton is called the photic zone. Below the photic zone, where light is attenuated to less than 1%, is the profundal zone. This is dominated by heterotrophic organisms subsisting on food that falls from above.

Deep waters may also be physically divided by thermal stratification in the winter and summer (Figure 15.4). Because water has a maximum density at 4°C, colder water in the winter and warmer water in the summer form a distinct layer that floats. The upper layer is called the epilimnion, the lower layer is the hypolimnion. Each of these layers is fairly uniform and, especially in the epilimnion, well mixed. These layers are separated by an area of rapid temperature and density change called the thermocline. The density gradient is very stable and prevents mixing between the two layers. The thermocline protects the profundal zone from the impacts of pollution. The littoral zone, on the other hand, is more susceptible because of its proximity to sources of pollution. The stratification shown in Figure 15.4 is typical of sufficiently deep lakes in summer in the temperate zone. Stratification varies significantly with season and local climate.

The basic characteristic of riverine (river or stream) environments is the velocity of flow. Riffles are relatively fast, shallow, and turbulent, whereas pools are laminar in flow, approaching lakes in their characteristics. Rivers generally lack the stratification found in lakes. Flow greatly affects dispersal of organisms and import and export of nutrients. Systems (whether rivers or lakes) that depend on their own productivity are called autochthonous. Those that depend on imported energy or nutrients are allochthonous. Lakes tend to be autochthonous. Rivers are more likely to be allochthonous, obtaining their organic matter from terrestrial sources such as leaf litter and other detritus in runoff.

Figure 15.4 Lake zones classified by depth, light, and stratification. (Based on Horne and Goldman, 1994.)

Littoral zone A

Pelagic zone

Figure 15.4 Lake zones classified by depth, light, and stratification. (Based on Horne and Goldman, 1994.)

Littoral zone A

Pelagic zone

508 ECOSYSTEMS AND APPLICATIONS 15.2.2 Biota

Aquatic organisms are classified according to their habitats. The plankton are small freefloating organisms. Planktonic plants, called phytoplankton, include cyanobacter, green algae, and diatoms. Planktonic animals are zooplankton and include protozoans, small crustaceans such as Daphnia species and larval forms of other animals. Organisms such as duckweed that float on the surface are called neuston. Active swimmers that live in the pelagic zone, such as fish, are called nekton. The community of microscopic bacteria, fungi, and small plants and animals that live attached to submerged surfaces such as rocks are referred to as aufwuchs. A subset of these are the periphyton, which are algae that grow attached to surfaces. (Aufwuchs and periphyton are what makes submerged rocks so slippery to step on.) All organisms that live on the bottom or within the sediments, whether in the littoral or profundal zones, are called benthic organisms, or simply the benthos.

Lotic ecosystems have planktonic organisms only in the slowest of rivers. Periphyton dominate over phytoplankton, and benthos over zooplankton.

Prokaryotes Three types of autotrophic bacteria are typically found in lakes: cyanobac-ter, chlorobacter, and chemoautotrophs. Cyanobacter (blue-green bacteria) are found in the epilimnion. Chlorobacteria peaks just below the thermocline, their numbers decreasing with depth. If the hypolimnion becomes anoxic, sulfate-reducing bacteria will appear to produce hydrogen sulfide. However, most of the bacteria are heterotrophs. Heterotrophic concentrations peak in the thermocline and near the sediment. In highly polluted streams the filamentous bacteria Sphaerotilus can form lush gray mats waving in the current.

One of the notable roles of cyanobacter, especially Aphanizomenon, is in the fixation of atmospheric nitrogen. This allows it to grow when nitrogen limitations apply to other algae. Some cyanobacter, such as Oscillatoria or Microcystis, cannot fix N2. The latter depends on ammonia recycled by animal or bacterial excretion or by solubilization from the sediments. In rivers, Nostoc or Rivularia may fix considerable amounts of nitrogen.

Gas vacuoles enable cyanobacter to float to the surface during the calm night, where they can be found forming a scum at dawn. As the sun rises higher, the cyanobacter can be damaged by ultraviolet radiation, so they tend to sink by midmorning. Anabaena does this by collapsing gas vesicles by osmotic pressure which has increased as a result of glucose formed by photosynthesis. Colonies of Microcystis and Aphanizomenon, as well as filaments of Anabaena, also regulate their buoyancy by manufacturing glycogen, which is about 1.5 times as dense as normal cell material. At night they use up the glycogen and start to rise again. By moving up and down, the algae not only control their light exposure but take advantage of nutrients farther below the surface.

Cyanobacter (Figure 15.5) make chemicals that inhibit feeding by zooplankton. They make compounds that have been known to poison cows and pigs that drank from eutrophic sources of water. Cyanobacter and some actinomycetes also produce geosmin and methylisoborneol, the most common causes of taste and odor problems in drinking water. These compounds are contained in the cells, but released when the cells break down.

Green Plants Algae are plantlike protists (Figure 15.6). They are distinguished from plants in that they do not have stems, roots, or leaves. They can be unicellular, colonial

Figure 15.5 Cyanobacter: (a) Aphanizomenon; (b) Anabaena; (c) Oscillatoria; (d) Polycystis; (e) Nostoc; (f) Spirulina. (From Standard Methods, 12th ed. © 1965 American Public Health Association.)

(forming usually small groups of cells), or filamentous. Some, such as the dinoflagellates, are motile, enabling them to migrate in response to light. Periphyton are usually green algae, cyanobacter, or diatoms. The long, bright green filaments attached to rocks in streams are often cyanobacter. Other filamentous forms are green algae such as Ulothrix and Cladophora.

Diatoms (Figure 15.7) have an advantage over other plants in that their cell wall, being made of silica, requires about 12 times less energy to manufacture than cellulose. (Cya-nobacter, which are prokaryotes, make cell walls of peptidoglycan, which requires more energy than cellulose. The ability of cyanobacter to control their light input by controlling buoyancy compensates them for this added cost.) However, diatoms are denser as a result, so under low-turbulence conditions they may settle out of the photic zone and be replaced by cyanobacter or dinoflagellates.

Figure 15.6 Green algae: (a) Pediastrum; (b) Selenastrum; (c) Coelastrum; (d) Scenedesmus; (e) Spirogyra; (f) Microspora; (g) Ulothrix. (From Standard Methods, 10th ed. © 1955 American Public Health Association.)
Figure 15.7 Diatoms: (a) Surirella; (b) Asterionella; (c, d) Gomphonema; (e) Tabellaría; (f, g) Navícula; (h) Eunotia; (i) Cymbella; (j) Fragilaria; (k) Gyrosigma. (From Standard Methods, 10th ed. © 1955 American Public Health Association.)

Dinoflagellates (Figure 15.8) can swim several meters per hour. They use their motility to swim upward in the morning to collect more light for photosynthesis, and then downward in the afternoon to avoid predation. In a condition called the red tide, dinoflagellates sometimes accumulate to nuisance levels in lakes, estuaries, and oceans, coloring the surface of the water blood-red. They can be toxic to fish and invertebrates and irritating to the skin of humans exposed by swimming. Examples of red tide genera include Noctiluca and Gymnodinium in marine systems and Peridinium and Ceratium in lakes.

In the open sea, the condition has been associated with salinity fronts where estuarine water converges with ocean currents. This leads to stratification between the estuarine water on top and the more saline water below. These waters may be complementary in the sense that each contains nutrients that may be limiting in the other. Algae grow preferentially at the boundary, where there will be some mixing. Dinoflagellates have the advantage of being able to move between the two layers, taking advantage of both.

Large plants are called macrophytes. Most are from the plant kingdom, although some are large algae (Table 15.4). Roots may be used for obtaining nutrition from the sediment or may be primarily for attachment. Nutrients can be obtained directly from the water. Besides their obvious role as primary producers, macrophytes are important as shelter and substrate for other organisms. Periphyton, which in this case are considered epiphytic, grow on the stems and leaves of the macrophytes. Protozoans roam the epiphytic cultures, and snails graze among them. Amphibians and small fish find shelter from predatory fish and birds. The macrophytes contribute greatly to the detritus food chain as dead plants settle to the bottom. Freshwater angiosperms will grow only at shallow depths. Deeper plants consist of ferns, mosses, and large algae. Emergent plants are those that grow

Figure 15.8 The dinoflagellates Peridinium and Ceratium.

out of the water. Some plants have only reproductive parts that are emergent. Many plants that are primarily emergent occur in marshes and are discussed in Section 15.3.

Zooplankton and Invertebrates Aquatic ecosystems can have all the trophic levels at microscopic size, as we are accustomed to seeing on a large scale in terrestrial ecosystems. The zooplankton include protozoans, small crustaceans, rotifers, and other small invertebrates (Figures 15.9, 15.10, and 15.11). These may have roles of herbivore and primary and secondary carnivore. The nymph or larvae of many insects are aquatic and have similar roles.

Protozoans are widely dispersed, and tend to be most abundant in waters with large amounts of organic matter (Table 15.5). They feed on detritus and other single-celled organisms, whether bacteria, algae, or other protozoans. Some are obligate parasites, with hosts ranging from algae to fish. The presence of parasites can change the species composition of an aquatic ecosystem, such as by bringing an algal bloom to an end.

Rotifers are among the more interesting creatures to observe under a microscope as they move and feed. Most are sessile (nonmotile); many are motile but attach themselves

TABLE 15.4 Common Aquatic Macrophytes

Habitat

Taxonomic Group

Examples

Free-floating

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